"H series" Bare minimum for sandy bridge CPU's. Cheapest. Doesn't support overclocking, SLI/crossfire, multiple monitors and rarely support multiple hard drives.
"P series" doesn't support integrated graphics (can be bad if something goes wrong with your video card) supports overclocking, SLI/crossfire, multiple Hard drives, not sure about monitors. I find this series to be worthless, as they don't seem to be any cheaper than the "Z series"
"Z series" most expensive, supports integrated as well as dedicated graphics, pretty much everything so far.
anything below a 77 will not support Ivy bridge without updates.
There are exceptions to everything... you can find H boards that can overclock, etc. Even so, they are either rare or a pain to do.